1. The Field of the Invention
The invention relates to a method for the environmentally friendly melting and refining of a glass melt for a crystallizable glass that can be used to prepare a lithium aluminium silicate (LAS) glass ceramic.
2. Description of the Related Art
When glass is melted, considerable amounts of gases arise as a consequence of the chemical conversion of the starting raw materials, of the batch. In the case of conventional cost-effective batch mixes, approximately 1.3 kg of batch raw materials are required for producing approximately 1 kg of glass. This means that considerable amounts of gases such as H2O, O2, CO2, SO2, NOx, N2 and air included in the batch are liberated during the melting-down process. The method step for removing the gas bubbles from the glass melt is referred to as refining.
When the batch is introduced into the melting tank, a batch heap forms on the glass melt and spreads into the melting tank to different extents as a so-called batch carpet.
When the batch is heated, a wide variety of reactions proceed which lead to glass formation. A description of these reactions may be found in the book “Allgemeine Technologie des Glases, Grundlagen des Schmelzens und der Formgebung” [“General Technology of Glass, Principles of Melting and Shaping”], Prof. Dr. H. A. Schaeffer, Erlangen September 1985. These reactions are generally subdivided with increasing temperature into:                dehydration        solid-state reactions in the region of the grain contacts (e.g. silicate formation)        formation of carbonate melts which envelop the quartz grains        decomposition reactions which produce bubbles (CO2, NOx, O2, SO3)        formation of silicate melts.        
The remaining constituents of the batch subsequently dissolve in the silicate melt.
Under the temperature conditions of the melting-down of the batch, these reactions mentioned proceed more or less alongside one another. The principal amount of the gases escapes during the batch reactions and the formation of the unrefined melt through the covering layer of batch.
These reaction steps also proceed in the case of conventional raw material mixes for the batch of the crystallizable glasses of lithium aluminium silicate (LAS) glass ceramics. Main constituents here are generally quartz sand as source of the SiO2 glass component, aluminium oxide or aluminium trihydroxide as source of the Al2O3 component and lithium carbonate as source of the Li2O component. Furthermore, the batch generally contains nitrates in order to set the oxidation state. When the temperature of the batch is increased, this gives rise firstly to release of water, then to nitrate decomposition and subsequently to liquid phase formation. What is crucial for the melting-down is the formation of a eutectic from the main constituents Li2O and SiO2 at approximately 1030° C. In this first liquid lithium-rich silicate phase, the remaining crystalline raw materials such as aluminium oxide, quartz sand, zirconium, the refining agent, and also some of the remaining gases such as, for example, O2, CO2, NOx, N2 and SO2 start to dissolve. As the remaining crystalline raw materials increasingly dissolve in the liquid phase, the gas solubility of the liquid phase decreases and buble formation occurs. In this case, the bubbles grow or shrink if the bubble internal pressure is lower or higher than the equilibrium pressure of the dissolved gases. Therefore, during refining, dissolved gases have to be extracted or reduced to a level that is no longer disturbing.
The dissolved gas residue in the end product is crucial for the reboil and should therefore be as small as possible.
Quartz sand and zirconium silicate and/or zirconium oxide are the last batch raw materials which are dissolved in the glass melt. They are those raw materials which determine the melting time and in the case of which, at excessively high tank throughputs, there is the risk of batch remnants. The dissolution rate is low in the case of LAS glasses and adhering bubbles bring the crystalline phases to the surface of the glass melt. The formation of surface layers composed of residual quartz and/or—formed from the latter at high temperatures—cristobalite (SiO2) and baddeleyite (ZrO2) is particularly pronounced in the case of aluminosilicate glasses.
The nature of the progression of the melting-down with targeted batch raw material selection, formation of the batch carpet and temperature of the glass in the melting-down region thus has crucial consequences for all subsequent sub-steps of glassmaking through to the product quality. If the melting rate during the melting-down process, as a result of excessively high tank throughput, is not coordinated with the removal at the processing end, quality problems (batch remnants, bubbles) occur in the glass. The sparingly soluble batch raw materials pass via the surface layer or via the depth flow into rear regions of the melting tank. As they dissolve, the gas solubility decreases in the chemically altered zone around the batch grains and the described effect of bubble formation occurs. The dissolving residual quartz grains are foreign seeds for continual new formation of bubbles (Nölle, Günther, Technik der Glasherstellung [Glassmaking Technology], Deutscher Verlag für Grundstoffundustrie Stuttgart 1997, 3rd edition, page 83). Micrographs showing such bubbles at the edge of dissolving residual quartz particles verify this mechanism. Such bubbles, which are produced in a late stage of the melting process, are virtually impossible to remove from the glass melt.
A targeted selection of the batch raw materials therefore has the potential to reduce the size of the batch carpet and the formation of the surface layers. By reducing the sparingly soluble batch raw materials, the risk of batch remnants and late bubbles is reduced. As is known, measures serving for homogenization always also make a contribution to refining, and vice versa. A batch raw material which comes close to the composition of the desired glass is therefore advantageous. Therefore, glass cullets which arise during production as well are added to the batch.
A reduction of the batch carpet and of the surface layer formation also promotes the heat input—required for increasing the temperature of the glass melt—by means of the infrared radiation from the gas burners and by means of reflection from the crown of the superstructure of the tank.
In a typical melting tank, the refining of the glass melt is carried out in a method step that temporally succeeds the melting-down and in a spatially separated region. The two regions are separated by the so-called hot spot of the melting tank. The hot spot is the point where the glass melt is at the highest temperature, that is to say that an upwardly directed flow of the glass melt takes place. As is known, various built-in structures are used in the design of typical melting tanks:                overflow wall for avoiding short-circuit flows and for obtaining a temperature increase as a result of low glass level height        bridge wall for avoiding short-circuit flows, primarily in the surface, and avoiding backflows        gas burners, usually arranged in the transverse direction, which emit their heat by radiation from the flame or by reflection at the crown into the glass melt        electrical additional heating for increasing the average glass temperature and flow stabilization        bubblers arranged transversely and/or longitudinally with respect to the glass flow direction for avoiding short-circuit flows, for flow stabilization and for increasing the average glass temperature by transporting cold bottom glass to the hot surface.        
In general, the hot spot in the tank is spatially fixed by the energy distribution (setting of the gas burners and of the electrical additional heating) or by additional structural measures such as an overflow wall, bubblers or electrical additional heating at the bottom of the tank.
The bridge wall is particularly suitable for counteracting the advance of the surface layers which are critical in the case of LAS glasses into the rear refining and standing region of the melting tank.
The glass level height should be adapted to the infrared transmission of the glass melt. Generally, an excessively large glass level height should be avoided in order to prevent cold zones in the bottom, which contributes to increasing the average glass temperature.
The effects of the built-in structures on the flow conditions in the melting tank are described and illustrated pictorially e.g. in the book by Nölle already cited (page 87 et seq.).
Refining involves assisting the gas bubbles in their endeavor, as a result of their static buoyancy owing to the difference in density between gas bubbles and glass melt, to ascend in the glass melt and then to escape into the open. Without supportive refining measures this process requires a considerable time, however, which would make the production process expensive owing to long stoppage times and low tank throughput resulting therefrom.
For LAS crystallizable glasses, various methods have developed as refining methods in a known manner.
In particular arsenic oxide and antimony oxide in contents of 0.3 to 1.5% by weight have proved worthwhile as chemical refining agents for LAS crystallizable glasses. These refining agents liberate O2 gases even at conventional refining temperatures of around 1600° C. or less in the glass melt, which O2 gases pass into the gas bubbles by diffusion. The gas quantities thus additionally passing into the gas bubbles lead to the desired bubble growth and thus to the desired increased rate of ascent of the gas bubbles. The ascending gas bubbles promote the homogeneity of the glass melt and counteract the surface layer. These refining agents are compatible with the required glass ceramic properties and lead to good bubble qualities of the melt.
Even if these substances are fixedly bound in the glass skeleton, they are still disadvantageous from safety and environmental protection standpoints. Special precautionary measures have to be taken during raw material procurement and preparation and owing to evaporation in the melt.
The search for alternative chemical refining agents which are less hazardous from environmental points of view has led to the use of tin oxide. The substitution of environmentally harmful arsenic oxide or antimony oxide by tin oxide alone is not readily possible owing to inadequate bubble qualities for economic tank throughputs. Owing to the low solubility of the tin oxide in the LAS crystallizable glasses, the maximum content is limited to values of less than 0.6% by weight or less. Devitrification otherwise occurs during shaping, owing to the low solubility. The Sn-containing crystals formed during devitrification adversely affect the strength of the glass and of the glass ceramic produced therefrom. Higher refining agent concentrations, as during chemical refining using arsenic oxide or antimony oxide, are therefore not possible. Furthermore, tin oxide liberates the oxygen required for refining in sufficient amounts only at relatively high temperatures. This reduces the efficiency of the use of tin oxide as a refining agent at customary conventional melting temperatures of up to 1700° C. The favorable effect of homogenization of the glass melt, which counteracts the formation of surface layers, is also less pronounced owing to the small amounts of the liberated O2 refining gas.
Therefore, ways have been sought for intensifying the refining effect of tin oxide by means of additional measures.
In order to achieve good bubble qualities, further refining agents are used alongside tin oxide, for example at conventional melting and refining temperatures (max. 1700° C.). A number of documents describe the use of halide compounds as additional refining agents.
Thus, the Japanese applications JP 11 100 229 A and JP 11 100 230 A describe the use of 0.1-2% by weight of SnO2 and 0-1% by weight of Cl. According to these documents, coloration is performed by adding V2O5 as sole colorant.
The addition of 0.05-1% by weight of fluorine (US 2007 0004578 A1) and 0.01-1% by weight of bromine (US 2008 0026927 A1) for supporting the refining using SnO2 is likewise disclosed. The main colorant is V2O5.
The addition of the halide compounds is disadvantageous since they evaporate to a large extent at the melting temperature and in the process form toxic compounds, such as e.g. HF or HCl. Furthermore, these compounds chemically attack the refractory bricks in the crown of the melting tank and corrosion occurs.
The document US 2007 0129231 A1 describes the combined use of 0.15 to 0.3% by weight of SnO2 in combination with 0.7 to 1.5% by weight of CeO2 and/or MnO2 as refining agents. Compared with refining by means of As2O3, these refining agent combinations yield distinctly poorer bubble qualities despite the comparatively high contents of CeO2 and/or MnO2. This is due to the fact that CeO2 and MnO2 cleave the oxygen required for refining at comparatively low temperatures and are less effective for the refining of the high-melting LAS crystallizable glasses.
Since the refining agent tin oxide releases the oxygen required for refining in relatively large amounts only at relatively high temperatures starting from approximately 1630° C., high-temperature refining above 1700° C. is appropriate for achieving good bubble qualities.
Thus, DE 199 39 771 B4, for example, discloses producing relatively high temperatures of between 1700° C. and 2800° C. by means of separate high-temperature refining units disposed downstream of the melting tank using radio-frequency and skull technology, in order thus to reduce the viscosity of the melt and hence to increase the rate of ascent of the gas bubbles.
In this case, two independent refining units connected to one another are typically provided.
WO 02/16279 A1 (=DE 199 39 787 C2), too, describes, inter alia, the production of a lithium aluminium silicate (LAS) glass ceramic colored with V2O5 in conjunction with reducing agents by means of high-temperature refining at 1975° C. for 1 h without the standard refining agents arsenic oxide or antimony oxide, but rather with alternative refining agents such as SnO2, CeO2, sulphate or chloride compounds. This glass ceramic, which appears black in plan view is typically used for the production of cook tops and is commercially available under the brand designation CERAN SUPREMA®.
These additional high-temperature refining units require capital expenditure on specific units and a different distribution of the energy input.
A further physical refining method is so-called vacuum refining. By way of example, reference should be made in this respect to EP 0 908 417 A2. The bubbles present in the melt likewise grow in the case of vacuum refining. The bubbles become larger as a result of these effects, ascend to the surface of the melt more rapidly and can leave the latter into the overlying furnace space.
Complex constructions are required for this method.
DE 10 2005 039919 A1 describes a method for refining a glass melt for a glass ceramic green glass, and a melting tank embodied accordingly. Provision is made of a glass batch on the basis of a lithium aluminium silicate (LAS) glass system with a sole addition of tin oxide as refining agent having a content of <0.4% by weight, while dispensing with arsenic oxide and/or antimony oxide as refining agent. The melting-down of the batch and refining of the melt are carried out at temperatures of <1700° C. while dispensing with additional specific high-temperature refining units. Depending on the tank construction, the refining agent content and the average glass temperature, a minimum residence time of the glass to be refined in the tank arises for the required bubble quality. In this method, too, the disadvantages of pure SnO2 refining at conventional refining temperatures are manifested in the limited tank throughputs. The document describes various melting tank designs which are also taken into account in the present invention.